feat: Introduce MediaAssetVariant for better asset management

- Added MediaAssetVariant enum with values 'main' and 'preview'.
- Updated media asset service logic to utilize MediaAssetVariant for variant handling.
- Refactored database models and queries to include variant and source_asset_id fields.
- Enhanced validation for asset variants in upload and processing functions.
- Updated Swagger documentation to reflect new variant structure and descriptions.
- Implemented necessary database migrations to support the new variant constraints.

backend/app/http/tenant/dto/media_asset_admin.go

@@ -1,6 +1,10 @@
 package dto
 
-import "time"
+import (
+	"time"
+
+	"quyun/v2/pkg/consts"
+)
 
 // AdminMediaAssetUploadInitForm defines payload for tenant-admin to initialize a media asset upload.
 type AdminMediaAssetUploadInitForm struct {
@@ -10,7 +14,7 @@ type AdminMediaAssetUploadInitForm struct {
 
 	// Variant indicates whether this asset is a main or preview product.
 	// Allowed: main/preview; default is main.
-	Variant string `json:"variant,omitempty"`
+	Variant *consts.MediaAssetVariant `json:"variant,omitempty"`
 
 	// SourceAssetID links a preview product to its main asset; only meaningful when variant=preview.
 	SourceAssetID *int64 `json:"source_asset_id,omitempty"`